Casting technique using purge gas and heat stabilizer

ABSTRACT

A DEFINE CLASS OF N,N,N&#39;&#39;,N&#39;&#39;-TETRAPHENYL-SUBSTITUTED P-BENZOQUINONE DIIMONIUM SALT INFRARED ABSORBERS ARE SUCCESSFULLY INCORPORATED INTO POLYMETHYLMETHACRYLATE BY AN IMPROVED CASTING TECHNIQUE IN WHICH THE POLYMER SYRUP IS PURGED WITH AN INERT GAS, SUCH AS NITROGEN, IN THE PRESENCE OF A P-BENZOQUINONE COMPOUND, SUCH AS CHLORANIL, PRIOR TO BEING POURED INTO THE HEAT-CURING CELL.

United States Patent M 3,557,052 CASTING TECHNIQUE USING PURGE GAS AND HEAT STABILIZER Robert Jerome Tucker, Hackettstown, N.J., assignor to American Cyanamid Company, Stamford, Conn., a corporation of Maine No Drawing. Filed Mar. 18, 1969, Ser. No. 808,302

Int. Cl. C08f 45/58 US. Cl. 26045.75 4 Claims ABSTRACT OF THE DISCLOSURE A defined class of N,N,N,N'-tetraphenyl-substituted p-benzoquinone diimonium salt infrared absorbers are successfully incorporated into polymethylmethacrylate by an improved casting technique in which the polymer syrup is purged with an inert gas, such as nitrogen, in the presence of a p-benzoquinone compound, such as chloranil, prior to being poured into the heat-curing cell.

This invention relates to a method for the incorporation of infrared absorbers into cast plastic material. More particularly, it relates to a method for the incorporation into cast polymethylmethacrylate of infrared absorbers of the following formula:

where each R, individually, represents a lower (C to C alkyl group and A represents an anion. The anion of the salt is not critical and may be any of a variety of anions, such as chloride (Cl), chlorate (CIO4 nitrate (NO sulfate (S05), benzenesulfonate (C H SO fiuoborate (131 hexafluoroarsenate (ASF hexafiuoroantimonate.

The infrared absorber salts, per se, are disclosed and claimed in a copending application, Ser. No. 692,312, filed Dec. 21, 1967, the disclosure of which is hereby incOrporated in the present application by reference.

The infrared absorbers of the above formula are the only infrared absorbers known to impart a desirable neutral shade of gray to cast sheet from polymethylmethacrylate. In a copending application of the present applicant, it was shown that this class of infrared absorbers suffer deterioration when subjected to the elevated temperatures encountered when they are incorporated into plastic substrates by common techniques such as injection moulding or extrusion. It was also shown that certain compounds, such as chloranil, are effective heatstabilizers for the absorbers, whereby they may be incorporated into plastics by the said methods without degradation.

When it is desirable to incorporate the absorbers into polymethylmethacrylate by conventional monomer casting techniques, the compounds degrade in a similar manner. However, the stabilizers described in the copending application are ineffective in preventing the degradation.

In another copending application of the present apand 3,557,052 Patented Jan. 19, 1971 plicant a method is described for incorporating infrared absorbers represented by the formula:

Formula I above into polymethylmethacrylate by casting techniques. That method involves a purging of the polymer syrup with an inert gas (nitrogen) prior to pouring it into the casting cell. When the purging technique is applied using the absorbers of Formula I, however, it fails to prevent degradation of the compounds.

Since there is a need for a method of incorporating the absorbers of Formula I into cast polymethylmethacrylate because of the desirable neutral shade of gray which they produce, a method for accomplishing this would constitute a valuable contribution to the art.

It has now been discovered that the infrared absorbers of Formula I can be incorporated into polymethylmethacrylate by casting techniques by the use of inert gas purging in the presence of p-benzoquinone compounds represented by the formula:

where each X, individually, represents hydrogen, chlorine, or fluorine. Representative compounds are, for example, benzoquinone, chloranil, fluoranil, and the mono-, di-, and trichloro or fluoro-p-benzoquinones.

It was surprising to find that neither the use of the gas purging alone nor the use of chloranil as a stabilizer alone would accomplish the desired result and that a combination of both is required.

In accordance with the invention, a casting mixture comprising low molecular weight polymethylmethacrylate syrup, catalyst, infrared absorber, and stabilizer is deaerated by bubbling an inert gas, such as nitrogen, through the mixture. The deaerated mixture is then degassed to prevent bubbles in the cast sheet and the degassed syrup is poured into a suitable casting cell under an inert gas blanket and cured by heating.

The method of the invention permits the use of infrared absorbers of Formula I in cast polymethylmethacrylate or copolymers of methylmethacrylate with other copolymerizable vinyl monomers normally used to make clear plastic sheet, which, in turn, is used in such applications as sun visors, welding windows, and glazing materials where the incorporation of infrared absorbers is needed or desirable. Included among the copolymerizable vinyl monomers are ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, ethylene glycol dimethacrylate, neopentyl glycol dimethacrylate and the like.

The castings are made using polymer syrups. It is preferred that the syrup be composed of partially polymerized polymethylmethacrylate (viscosity in the range of about 400 to 800 centipoises) and a small amount of catalyst, preferably azobisisobutyronitrile. The method is also applicable to syrups composed of polymethylmethacrylate, methyl methacrylate monomer, and catalyst. If desired, the casting maybe made using methyl methacrylate monomer with catalyst. The syrup may also contain a lubricant, ultraviolet absorbers, dyes, pigments, other infrared absorbers, photochromic materials, stabilizers and the like.

At least about 0.01% by weight of the infrared absorber is used in the casting mixture, the usual amount being from about 0.01% to about 1.0%. Higher amounts, however, may be used.

The amount of stabilizer used must be at least equal to the amount, by weight, of the infrared absorber; an excess of from about to about 20, or more, times the weight of the absorber being generally preferred.

Once the casting syrup has been deaerated and degassed according to the method described, it is poured into a casting cell of the desired dimensions and cured. The curing is generally conducted at a temperature in the range of about 55 C. to 70 C. for from 3 to 16 hours, depending on the thickness of the cell. In addition, after curing the polymer, it may be subjected to a postcuring cycle at a higher temperature.

Using this method, cast sheet is obtained having very high absorption in the infrared from about 700 to about 1600 nanometers, and a desirable neutral shade of gray.

The invention is illustrated by the following examples.

EXAMPLE 1 The following casting mixture was prepared:

G. Polymethylmethacrylate syrup 1400 N,N,N',N tetrakis(pdibutylaminophenyl)-p-ben- Half of the above mixture was deaerated by sparging with nitrogen, degassed in vacuo, and cast into a 18" X 18" x 0.1 cell. The cell was heated at 70 C. for 2.5 hours (casting A). The other half of the mixture was degassed under vacuum and cast into a similar cell and cured under the same conditions (casting B). After the castings were removed from the cells, a spectrum was obtained and an analysis made of the absorption at 1060 nanometers. Casting A had an optical density at this wavelength of 4.25, whereas casting B had an optical density of only 2.7, thus showing that casting B, which was not purged with nitrogen, had lost infrared absorber due to degradation.

EXAMPLE 2 The following casting mixture was prepared:

G. Polymethylmethacrylate syrup (550 cps.) 100 N,N,N',N tetrakis(p-dibutylaminophenyl)-p-benzoquinonebis(imonium hexafiuoroantimonate) 0.052

Azobisisobutyronitrile 0.2

4 of the infrared absorber. The optical density at 1060 nanometers was about 2.0 whereas if no degradation occurred the optical density is about 4.5.

The example illustrates that inert gas purging alone is not sufficient to stabilize the infrared absorber in the casting process.

I claim:

1. A casting method for incorporating an infrared absorber, represented by the formula:

R R R R wherein each R, individually, represents lower alkyl and A is an anion, into polymethylmethacrylate which comprises (a) purging a Polymethylmethacrylate polymer syrup, containing at least about 0.01% by weight of the infrared absorber and at least an equal amount of a stabilizer compound represented by the formula:

References Cited UNITED STATES PATENTS 11/1968 Carroll et al 264l02 12/1969 Susi et al. 26045.75

OTHER REFERENCES Journal of Polymer Science, vol. XIX (1956), pp. 123- 140, article on Methylmethacrylate Polymerization by Kice.

DONALD E. CZAJA, Primary Examiner V. P. HOKE, Assistant Examiner US. Cl. X.R. 

